Many People know answer for this , but some people are there who are new into this,
What is UNION?
UNION is used to join two related set of data.UNION will give a distinct result ie; it
will avoid duplicates.How ever the UNION is bit slower due to distinct sort.
What is UNION ALL
UNION ALL is also used to join two related set of data.UNION ALL will not Provide a distinct sort.So it is very fast than UNION